This invention relates to drive systems for use with electric motors and, in particular, to a drive system for use with variable speed electric motors utilized in household appliances.
Electric motors are commonly used in various household appliances including, for example, washing machines. Many large appliances such as washing machines are characterized by having both a main motor drive and various system and interface control functions. In a washing machine, for example, the system control functions include timing the washing action, setting the speed of the main drive, controlling the water level, and controlling the detergent inlet. These functions all depend, for each wash cycle, on the selections made by the user on the appliance control panel.
The motor drive for washing machines must function efficiently and effectively over a wide speed range with very low noise. Long life, the ability to repeatedly start and stop, and the ability to reverse motion direction often are also requirements of washing machine motor drives. Over the years, attempts have been made to simplify the drive systems employed to drive the agitator and spin basket of these machines. Many motor types, including both induction motors and direct current motors of various constructions, have been used for this purpose. Recently, brushless permanent magnet motors and electronically controlled motors having unusual winding configurations have been suggested for use in such appliances. As a rule, motors of this type are expensive to manufacture and difficult to control. Typically, they require expensive and sophisticated electronic control circuitry.
Problems arise in trying to incorporate sophisticated motors into these types of appliances. Most operations, such as opening and closing valves, controlling pumps, etc. do not require high speed control circuitry. On the other hand, motor control must occur, at least in part, at a much faster rate. Consequently, the electronic system associated with the motor drive must accommodate both slow and high rates of operational control. While circuits capable of performing both slow and high speed control functions can be designed, their cost, together with the cost of the sophisticated electric motors, rapidly drives the cost of the appliance out of the range which most buyers are willing to pay.
A desirable speed control technique would use a variable frequency inverter in combination with an induction motor. Such a drive system would provide the benefits of a motor which has been well tested, is extremely reliable, and is low in cost. Further, an induction motor is capable of the performance required by virtually any domestic appliance. While this solution is attractive, there is a penalty since the electrical control for induction motors is very complex. Complexity in general translates into a higher cost of design. The functions performed by inverter drives are varied, including speed control of the motor, motor flux and slip calculations, calculation of acceleration and deceleration ramps and protectional limits during reversal of the motor, synthesis of appropriate values of voltage and frequency for motor drive control, control of the output frequency of the inverter, and control of the output voltage or current of the inverter.
Although many of these inverter control functions may be performed at a relatively slow rate, control of the output voltage and/or current from the inverter to the motor may not. That function must be accomplished at a very high rate. In a system utilizing a microprocessor, the inverter control function of controlling output voltage and/or current usually requires a disproportionate amount of microprocessor capacity. Particularly in the case of a low cost microcontroller, drive control at these high frequencies leaves almost no space capacity for the other inverter control functions, or for the system and interface control functions mentioned above.